1. Field of the Invention
The present invention relates generally to optimization of extreme ultraviolet (EUV) light generation, particularly closed-loop adjustment of optical element setpoints to optimize alignment of a laser beam onto a target droplet.
2. Description of the Prior Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to generate EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), the required plasma can be generated by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site within an LPP EUV source plasma chamber
The line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may be mixed or dispersed with another material such as a liquid. Delivering this target material and the laser beam simultaneously to a desired irradiation site within an LPP EUV source plasma chamber for plasma initiation presents certain timing and control problems, as it is necessary to hit the target properly in order to obtain sufficient plasma to maximize EUV light generation. In fact, the position of the laser beam focus relative to the droplets determines, in part, the power output of the EUV light source.
Thus, the laser beam must be focused on a focus position through which the target material will pass, and must be timed so as to intersect the target material when it passes through that point. In a three-dimensional space, drops of the target material travel along an x-axis and the laser beam travels along a z-axis (with a y-axis intersecting the x- and z-axes). The focus position of the laser beam is determined by two separate optical elements: a lens (the “final focus lens” or “FF lens”) and a steering mirror (the “final focus steering mirror” or “FF steering mirror”). To keep the laser beam focused on the focus position, an FF mirror (FFY) control loop algorithm directs alignment of the steering mirror to position the laser beam along the y-axis and an FF lens (FFZ) control loop algorithm calibrates lens alignment to position the laser beam along the z-axis.
Within current LPP EUV systems, these feedback control loops are used to dynamically track and adjust axial positioning of the FF lens and the FF steering mirror in order to control lens alignment. A sensor (e.g., a return beam diagnostic (RBD) camera) can determine a relative focal position—that is, where the beam is focused relative to the droplet. Thus, in theory, one can implement a feedback control loop receiving input from a relative focal position sensor to maintain the laser beam focused on the target material (e.g., droplet). The problem with a RBD approach, however, is that it can yield a biased measurement that can indicate movement of the relative beam-to-droplet alignment when, in reality, the alignment is stationary. Thus, control algorithms based on input from relative focal position sensors are of limited utility. What is needed, therefore, is an improved way to accurately track and adjust the position of each optical element so as to be able to maintain focus of the laser beam on the droplet.